Fourteen police officers could face prosecution over blunders during investigations into two serial sex attackers.
Suburban street stalker Kirk Reid, 44, and taxi driver rapist John Worboys, 51, were jailed earlier this year.
Reid preyed on women in streets near his home, while Worboys drugged and sexually assaulted some of his passengers.
Both cases highlighted serious shortcomings in the police inquiries, including bad decisions, poor communication between officers and weak witness care.
Officials from the Independent Police Complaints Commission (IPCC) launched two investigations in the wake of the guilty verdicts.
The results, widely anticipated to criticise overall police tactics for tackling sex crimes, are expected to be published before Christmas.
An IPCC spokeswoman confirmed seven officers from each case have been served so-called Regulation 9 notices.
The notices were issued before formal interviews and warned officers they were under investigation and could ultimately face legal action.
Those in the spotlight were mainly from junior ranks, but in the Reid case at least one officer of superintendent rank has been interviewed.
Reid, who preyed on 27 women, the majority on the streets of the Balham, Clapham and Tooting areas of south west London, was jailed for life in June.
After his conviction, police were criticised for leaving him free to target women for four years despite identifying him as a suspect.
A litany of police errors, inaction and oversights was revealed during the nine-week trial.
Police officers in Wandsworth identified that a serial sex attacker was on the loose after linking 25 assaults in September 2002.
But Reid slipped through the net several times despite arousing the suspicions of a series of officers.
Worboys was jailed indefinitely in April after a jury convicted him of drugging and sexually assaulting women passengers.
The trial highlighted how police failed to link his distinctive method of drugging and attacking his victims.
A total of 14 women came forward between 2002 and 2008 to report that they had been drugged and attacked by the driver of a black taxi.
But police did not link the crimes and several women were simply not believed because they had been drinking.
Senior Metropolitan Police officers decided to reorganise the way the force tackles sexual offences in the wake of the cases.
Borough Sapphire teams were brought under the control of a central team at the specialist crime directorate.